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From Proteopedia
Mouse importin alpha: mouse CBP80 cNLS complex
Structural highlights
Function[IMA2_MOUSE] Functions in nuclear protein import as an adapter protein for nuclear receptor KPNB1. Binds specifically and directly to substrates containing either a simple or bipartite NLS motif. Docking of the importin/substrate complex to the nuclear pore complex (NPC) is mediated by KPNB1 through binding to nucleoporin FxFG repeats and the complex is subsequently translocated through the pore by an energy requiring, Ran-dependent mechanism. At the nucleoplasmic side of the NPC, Ran binds to importin-beta and the three components separate and importin-alpha and -beta are re-exported from the nucleus to the cytoplasm where GTP hydrolysis releases Ran from importin. The directionality of nuclear import is thought to be conferred by an asymmetric distribution of the GTP- and GDP-bound forms of Ran between the cytoplasm and nucleus. [NCBP1_MOUSE] Component of the cap-binding complex (CBC), which binds cotranscriptionally to the 5'-cap of pre-mRNAs and is involved in various processes such as pre-mRNA splicing, translation regulation, nonsense-mediated mRNA decay, RNA-mediated gene silencing (RNAi) by microRNAs (miRNAs) and mRNA export. The CBC complex is involved in mRNA export from the nucleus via its interaction with ALYREF/THOC4/ALY, leading to the recruitment of the mRNA export machinery to the 5'-end of mRNA and to mRNA export in a 5' to 3' direction through the nuclear pore. The CBC complex is also involved in mediating U snRNA and intronless mRNAs export from the nucleus. The CBC complex is essential for a pioneer round of mRNA translation, before steady state translation when the CBC complex is replaced by cytoplasmic cap-binding protein eIF4E. The pioneer round of mRNA translation mediated by the CBC complex plays a central role in nonsense-mediated mRNA decay (NMD), NMD only taking place in mRNAs bound to the CBC complex, but not on eIF4E-bound mRNAs. The CBC complex enhances NMD in mRNAs containing at least one exon-junction complex (EJC) via its interaction with UPF1, promoting the interaction between UPF1 and UPF2. The CBC complex is also involved in 'failsafe' NMD, which is independent of the EJC complex, while it does not participate in Staufen-mediated mRNA decay (SMD). During cell proliferation, the CBC complex is also involved in microRNAs (miRNAs) biogenesis via its interaction with SRRT/ARS2 and is required for miRNA-mediated RNA interference. The CBC complex also acts as a negative regulator of PARN, thereby acting as an inhibitor of mRNA deadenylation. In the CBC complex, NCBP1/CBP80 does not bind directly capped RNAs (m7GpppG-capped RNA) but is required to stabilize the movement of the N-terminal loop of NCBP2/CBP20 and lock the CBC into a high affinity cap-binding state with the cap structure.[1] Publication Abstract from PubMedClassical nuclear localization signals (cNLSs), comprising one (monopartite cNLSs) or two clusters of basic residues connected by a 10-12 residue linker (bipartite cNLSs), are recognized by the nuclear import factor importin-alpha. The cNLSs bind along a concave groove on importin-alpha; however, specificity determinants of cNLSs remain poorly understood. We present a structural and interaction analysis study of importin-alpha binding to both designed and naturally occurring high-affinity cNLS-like sequences; the peptide inhibitors Bimax1 and Bimax2, and cNLS peptides of cap-binding protein 80. Our data suggest that cNLSs and cNLS-like sequences can achieve high affinity through maximizing interactions at the importin-alpha minor site, and by taking advantage of multiple linker region interactions. Our study defines an extended set of binding cavities on the importin-alpha surface, and also expands on recent observations that longer linker sequences are allowed, and that long-range electrostatic complementarity can contribute to cNLS-binding affinity. Altogether, our study explains the molecular and structural basis of the results of a number of recent studies, including systematic mutagenesis and peptide library approaches, and provides an improved level of understanding on the specificity determinants of a cNLS. Our results have implications for identifying cNLSs in novel proteins. Structural Basis of High-Affinity Nuclear Localization Signal Interactions with Importin-alpha,Marfori M, Lonhienne TG, Forwood JK, Kobe B Traffic. 2012 Jan 16. doi: 10.1111/j.1600-0854.2012.01329.x. PMID:22248489[2] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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